Paper products and method of making the same



United States Patent PAPER PRGDUCTS AND METHOD OF MAKING THE SAME Howard E. Shearer, Swarthmore, Pa., assignor to American Viscose Corporation, Wilmington, Del., 21 corporation of Delaware No Drawing. Application February 9, 1950,

Serial No. 143,355

5 Claims. (Cl. 92-3) This invention relates to paper products comprising randomly intermingled discontinuous fibers at least of which are non-fibrillating, synthetic or artificially formed fibers of paper-making length, i. e., fibers having a length of from one thirty-second to thirteen thirtyseconds of an inch, and to a novel method of making such papers.

By the term paper products is meant products comprising a multiplicity of the discontinuous fibers of papermaking length associated together to form a coherent product which may be flexible or stiff, thick or thin, soft or hard, and including paper for any purpose and especially for use as teabags, towelling, strong thin paper for use as backing for mimeograph stencils, kraft-type paper bags, boxboard, cardboard, Wallboard, insulation, tubes, gasmask filters, and molded paper articles of all kinds.

In the normal manufacture of paper, cellulose fibers, such as those derived from wood pulp, are beaten in water to disperse the fibers and to reduce them to a length and fineness suitable for paper-making. During the beating, the cellulose fibers fibrillate, the fibrillation manifesting itself by a fraying of the surfaces and ends of the fibers to produce minute tendrils or fibrillae which serve to felt or interlock the fibers together when they are deposited on the forming wire of the paper machine. The interlocking of the fibrillae projecting from the deposited fibers imparts coherency and strength to the paper. At the same time, the fibrillae effect an at least partial closing of the interstices between the fibers, thereby reducing the porosity of the paper.

Synthetic or artificially formed fibers, such as regenerated cellulose fibers for example are not naturally adapted to use in the manufacture of paper. Such fibers tend to form fiocs or clumps in water, which can be dispersed only with considerable diificulty. Furthermore, the synthetic or artificially formed fibers do not fibrillate when they are beaten in water, regardless of the severity of the beating or the beating time. Therefore, those fibers do not interlock or felt together when they are deposited from aqueous suspension on a paper-making screen. The presence of non-fibrillatin g, synthetic or artificially formed fibers in the paper will, under ordinary conditions, markedly decrease the strength of the paper. A paper comprising the normal fibrillating paper-making fibers and as much as 10% of regenerated cellulose fibers has been found to have low strength and poor resistance to tearing or bursting in both the wet and dry states.

Various bonding agents have been proposed heretofore for addition to a furnish comprising paper-making stock. Synthetic thermosetting resins of the melamine-formaldehyde type have been suggested. These have the disadvantage that the resin has to be cured on the paper. In addition, when the paper is reclaimed, the resin sets in the form of hard globules, or beads, under heat generated by friction in the digester, and the globules cannot be readily separated from the reclaimed stock. Other bonding agents proposed previously have been r ficial fibers having a length of from water-soluble, and therefore incapable of enhancing the Wet strength of the paper and even tend to decrease the wet strength. Still other bonding agents have been of a type which reduces the porosity of the paper. For example, it has been proposed to make a paper comprising synthetic or artificial fibers bonded together by fibrillae reclaimed from the wash water obtained by digesting and washing natural fibers, or fibrillae obtained by prolonged beating and abnormal hydration of the cellulose fibers normally used in making paper. These fibrillae remain in fiber or fine filament form when they are mixed with the synthetic or artificial fibers and occur as such in the finished paper, in which they can be discerned projecting from points of intersection of the synthetic or artificial fibers. The presence of these discernible fibrillae projecting from the fibers materially decreases the porosity of the paper by reducing the size of interstices between the fibers. A paper of regenerated cellulose fibers comprising even small amounts of these fibrillae is not as porous as a paper consisting entirely of the regenerated cellulose fibers.

One object of this invention is to provide coherent, strong, paper products comprising from at least 10% to of non-fibrillating synthetic or artificially formed fibers of paper-making length in which the non-fibrillating fibers are bonded together at their points of intersection by a discontinuous, essentially non-fibrous water-insoluble binder which does not reduce the porosity of the paper, the paper being characterized by high dry and wet strengths. Another object is to provide a method of making the paper products comprising from 10% to 100% of non-fibrillating synthetic or artificially formed fibers. A further object is to provide a new, essentially non-fibrous water-insoluble bonding agent for nonfibrillating synthetic or artificial paper-making fibers which is also a dispersing agent for the fibers in aqueous media. A further object is to provide paper products all of the fibers of which are non-fibrillating synthetic or artificial fibers of paper-making length bonded together at their points of intersection by a discrete Water-insoluble binder, the paper being substantially as porous as. if it consisted entirely of the non-fibrillating synthetic or artificial fibers.

According to a preferred modification of the invention, paper products having novel characteristics are made by dissolving a water-insoluble, alkali-soluble cellulose ether in an aqueous alkaline solution containing from 1 to 8% of a strong alkali, mixing the solution with water contained in the beater of a paper-making machine to disperse the ether in the form of very fine, colloidal gel particles in the Water in the beater, mixing a predetermined quantity of paper-making fibers at least 10% by weight of which are non-fibrillating synthetic or arti- /32 to inch with the resulting dilute aqueous alkaline dispersion of the ether, and either before or after precipitation of the ether, depositing the fibers from the dispersion to obtain a paper product of predetermined shape and thickness, and drying and heat-setting and pressing the product to obtain a paper product in which the non-fibrillating synthetic or artificial fibers are bonded together by the water-insoluble, alkali-soluble cellulose ether which occurs in the product in discontinuous condition.

The ether, which is insoluble in water and in aqueous alkaline solutions containing less than 1%, alkali, is dispersed in the dilute aqueous alkaline medium in the beater and occurs therein in the form of extremely fine, colloidal gel particles, and as I have found, the dispersed ether is an effective dispersing agent for the non-fibrillating synthetic or artificial fibers, so that those fibers can be uni formly dispersed in the aqueous medium containing the ether without the formation of clumps or flocs, and without subjecting the mass to a beating comparable to the beating to which the cellulose fibers conventionally used in making paper are subjected. When the fibers are all non-fibrillating synthetic or artificial fibers, therefore, the fibers can be dispersed in the aqueous medium without beating, the beater roll being used merely as a mixing device. When a stock comprising at least 10% of the non-fibrillating synthetic or artificial fibers in admixture with natural fibers, is to be prepared, the natural fibers may be beaten in the water prior to dispersing the waterinsoluble, alkali-soluble cellulose ether and the synthetic or artificial fibers therein, or the natural fibers may be beaten in water in a separate beater and then introduced into the aqueous alkaline medium containing the dispersed water-insoluble, alkali-soluble cellulose ether simultaneously with the synthetic or artificial fibers. In any event, whether the paper is to comprise fibers all of which are synthetic or artificial fibers, or mixtures of such fibers with natural, fibrillating paper-making fibers, it is important to disperse the water-insoluble, alkali-soluble cellulose ether in the aqueous alkaline medium before the synthetic or artificial fibers are introduced, in order to disperse those fibers uniformly in the medium and prevent the formation of fiber clumps or flocs.

It is preferred to precipitate the cellulose ether on the fibers in the beater, and this is accomplished by adding a solution of a suitable acid or acid salt, which does not decompose the ether, for example sulfuric acid, to the beater, after the synthetic or artificial fibers have been dispersed therein. The precipitation results in an agglomeration of the Water-insoluble, alkali-soluble cellulose ether gel particles into particles of larger size which become affixed to the fiber surfaces and adhere to the surfaces in the particle form. When the fibers having the discrete cellulose ether particles adhered to their surfaces are separated from their aqueous alkaline suspension, the fibers are deposited in intermingled, randomly distributed condition and held together in that condition by the cellulose particles at their points of intersection. Since the ether does not comprise fibrillae, interstices between the fibers remain open, and therefore, the presence of the cellulose ether in the discontinuous condition in the final product does not substantially decrease the porosity of the paper. If all of the fibers in the paper are non-fibrillating synthetic or artificial fibers which are bonded at their points of intersection by the discrete water-insoluble, alkali-soluble cellulose ether, the porosity of the paper is substantially the same as if the cellulose ether were not present and the paper consisted entirely of the synthetic or artificial fibers. Papers comprising fibrillated natural fibers in admixture with the non-fibrillating synthetic or artificial fibers are less porous than papers which do not comprise the fibrillated fibers, but are more porous than a paper comprising fibrillated fibers only.

The aqueous dispersion containing the water-insoluble, alkali-soluble cellulose ether and paper-making fibers at least 10% of which are synthetic or artificial fibers may be formed into a paper product by any suitable process. For example, it may be formed into a web or sheet by the use of machines of various types, such as the Fourdrinier, or Harper, single cylinder or Yankee multivat machine, mould, presse pate, or the like. Thick paper products may be obtained by passing a layer of fibers carrying the precipitated cellulose ether (which layer has been formed on a cylinder and couched to the felt of a cylinder paper-making machine, or run from the apron onto the wire and couched to the felt of a Fourdrinier paper-making machine) between press rolls with another layer of fibers carrying the precipitated ether, before the layers are completely dry, and repeating the procedure until a sufficient number of layers have been built up, and then subjecting the assembled plies to heat and pressure.

Instead of precipitating the water-insoluble, alkali-soluble cellulose ether on the fibers in the beater, it may be carried in the dispersed condition on the fibers through the forming or shaping steps and subsequently fixed on the fibers in the shaped paper product. For example, the ether may be fixed on the fibers in the shaped paper article by heat, in which case fixing of the ether, and final drying and pressing of the product, may be simultaneous. If fixing is by heat only, temperatures of from to C. may be employed. Alternatively the paper sheet or other paper article comprising the fibers carrying the dispersed cellulose ether may be partially dried, and then passed through or immersed in an aqueous acid or acid salt solution in which the ether is precipitated, or subjected to a suitable acid gas or vapor such as moist sulfur dioxide to precipitate the ether, subsequently washed free of acid, dried, and pressed.

In another embodiment of the invention formaldehyde is added to the water in the beater, in addition to the water-insoluble, alkali-soluble cellulose ether. In practicing this embodiment, it is preferred to dilute the alkaline solution of the ether prepared initially, for example with cold water, and to add the formaldehyde (as commercial formalin) and acid, such as sulfuric acid to the diluted solution prior to introducing it into the beater. The advantage of this embodiment is that the cellulose ethercontaining medium added to the beater has an acid pH, which may be desirable in some cases. No further acid treatment for precipitating the cellulose ether is required.

The term water-insoluble, alkali-soluble cellulose ether is intended to include ethers which are insoluble in 30v water but which are soluble in aqueous alkaline medium containing from 1 to 8%, preferably 6 to 8% of a strong alkali, for example sodium, potassium, or lithium hydroxide, at room temperature, or at reduced temperature. Included are simple alkyl ethers, carboxylalkyl ethers, hydroxyalkyl ethers, mixed alkyl hydroxyalkyl ethers, mixed alkyl carboxyalkyl ethers, and the salts of the carboxyalkyl ethers.

The proportion of water-insoluble, alkali-soluble cellulose ether added to the water in the beater may vary somewhat, but usually is such that the aqueous medium in the beater contains from 0.02% to 0.5% of the ether on the weight of the water. If the alkaline solution of the ether is added directly to the beater, the concentration of alkali in the beater is such that the dispersion of the ether has a pH of from 10 to 14.

When the water-insoluble, alkali-soluble cellulose ether is to be precipitated on the fibers in the beater, this may be accomplished by adding an aqueous solution of an acid or acid salt which adjusts the pH to the acid side and does not decompose the ether to the beater. Sufficient acid or acid salt is added at this point to give a furnish having a pH of 6 to 7 when the dispersion is diluted in the usual manner prior to sheeting or forming. Examples of suitable acids are sulfuric, acetic, boric, and hydrochloric acids. Examples of suitable acid salts are sodium dihydrogen phosphate, sodium bisulfate, ammonium sulfate, and aluminum sulfate. As illustrative, aqueous sulfuric acid solutions of from 0.2 to 0.4% concentration are satisfactory. If the acid solution is added to the beater, prior to diluting the paper-making stock, no special treatment is required to neutralize the acid, the concentration of acid being insufficient to damage any regenerated cellulose fibers present. However, if the acid is added after the stock has been diluted to the final desired fiber concentration, a larger amount of acid is required to bring the diluted mass to the desired pH and therefore it may be desirable to treat the diluted dispersion with an aqueous medium containing a salt, such as sodium carbonate, for neutralizing the acid and adjusting the pH to 6 to 7.

The synthetic or artificial fibers used in practicing this invention are of paper-making length, i. e., they have a length of from one-thirty second of an inch to thirteenthirty seconds of an inch. The fibers may be cut to the desired length, prior to being added to the aqueous alkaline dispersion of the water-insoluble, alkali-soluble cellulose ether.

By selection of the proportion of non-fibrillating synthetic or artificial fibers to natural fibers, if natural fibers are used, the length of the synthetic or artificial fibers within the range stated, and the proportion of water insoluble, alkali-soluble cellulose ether, the characteristics of the final paper product may be varied. Tough paper products having a substantial degree o-f flexibility and relativelysoft to the touch may be made, or the products may be relatively stiff and resilient, or they may have a high resistance to bursting or tearing in the wet state while remaining capable of being folded, bent, or shaped. The products are, in general, readily wettable, have high wet strength and are more porous than similar paper products consisting of fibrillated fibers or paper products in which synthetic or artificially formed fibers are bonded together by fibrilla-e.

The non-fibrillating synthetic or artificial fibers may be formed from any suitable fiber-forming material ineluding regenerated cellulose; cellulose acetate; casein; high molecular weight synthetic linear polyamides or polyesters; polymeric materials such as copolymers of vinyl chloride and vinylidene chloride, polyethylene, polyacrylonitrile, copolymers of acrylonitrile or its homologues with one or more other polymerizable substances containing the linkage C=C for example, copolymers of acrylonitrile with one or more of the following: vinyl acetate, vinyl chloride, styrene, isobutylene, etc. especially those acrylonitrile polymers containing at least 70% acrylonitrile in the polymer molecule.

The natural fibers which may be admixed with the synthetic or artificial fibers in a proportion of not more than 90% by weight inolude wood, rag fibers, cotton, flax, hemp, jute, straw, or other fibers derived from plants of various kinds and, in general, fibers of the type conventionally used in making paper.

The following examples are illustrative of the invention. In the examples, standard viscosity is the viscosity of a solution of the ether containing 85% water, 6% cellulose ether and 9% sodium hydroxide by weight, measured at 25 C. and expressed as a multip e of the viscosity of glycerol, also measured at 25 C.

Example I A water-insoluble, alkali-soluble hydroxyethyl cellulose having a standard viscosity of 6, and an average degree of substitution of 0.20 ethylene oxide group per glucose unit is dissolved in 8% sodium hydroxide to obtain a 6% solution of the ether, and the solution is mixed with water in the beater of a paper-making machine to obtain a dilute aqueous alkaline dispersion containing 0.2% (based on the weight of the water) of the ether in the form of very fine colloidal gel particles. 1.5% (based on the weight of the water) of regenerated cellulose fibers cut to a length of three-eighths inch are added. After thorough mixing (but without subjecting the mass to any beating action comparable to the beating to which cellulose fibers are normally subjected in making paper), willcient concentrated sulfuric acid of 1.84 specific gravity is added to give a final paper-making stock having a pH of 6-7 when the dispersion is diluted with water to 0.02%- 0.002% fiber content in the customary practice of preparing whole stock for the head box of the paper-making machine. The acid precipitates the fine colloidal particles of the cellulose ether as larger particles which adhere, as particles, to the surfaces of the fibers. The stock is sheeted on a Fo-urdrinier paper machine, the sheet is passed over drying cans at 145 C., and then to the calender rolls, between which it is subjected to a pressure of 1000 lbs/sq. in. The final pressed or calendered product is a light-weight, readily wettable, absorbent, porous paper sheet in which the regenerated cellulose fibers are bonded together at their points of intersection by the precipitated cellulose ether which occurs in the 6 product in discontinuous particulate condition. The paper is characterized by high dry and wet strengths, and is resistant to tearing or bursting on exposure to water, organic solvents, and aqueous media, except aqueous media containing sufficient alkali to dissolve the cellulose ether. The product is particularly well-suited to use as teabag paper.

Example II A water-insoluble, alkali-soluble hydroxyethyl cellulose having a standard viscosity of 2, and average degree of substitution of 0.36-0.40 ethylene oxide group per anhydroglucose unit, is dissolved in 8% sodium hydroxide to obtain a 6% solution of the ether. The solution is mixed with water in the beater of a paper-making machine to obtain a dilute aqueous alkaline dispersion containing 0.2% (on the weight of the water) of the ether in the form of extremely fine gel particles. Then, 1.5% (on the weight of the Water) of a mixture comprising 10% regenerated cellulose fibers having a length of threeeighths inch, and of prebeaten fibrillated cellulose fibers derived from wood pulp, is added to the dispersion and distributed therein, using the beater roll as a mixing device. The stock is diluted to 0.02% fiber content, and sheeted on a Fourdrinier machine. The sheet is partially dried and then passed through an aqueous solution of sulfuric acid which precipitates the water-insoluble, alkali-soluble cellulose ether. The sheet is then passed through a water bath to remove sulfuric acid, passed over drying cans at C., and then to calender rolls between which it is subjected to a pressure of 1000 lbs./ sq. in. The final product is a light-weight, thin paper sheet which is stronger in the wet state than a paper of similar weight and thickness but containing fibrillated fibersexclusively. At the same time, the paper is more porous than a paper in which all of the fibers are beaten, fibrillated, conventional paper-making fibers.

Example III A water-insoluble, alkali-soluble hydroxyethyl cellulose ether having a standard viscosity of 6, and an average degree of substitution of 0.36-0.40 ethylene oxide group per glucose unit, is dissolved in 8% sodium hydroxide to give a 6% solution of the ether. The solution is mixed with water in the beater of a paper machine to obtain a dilute aqueous alkaline dispersion containing 0.3% (based on the weight of the water), of the ether in the form of extremely fine gel particles. Then 2% (based on the weight of the water) of a mixture consisting of 30% regenerated cellulose fibers cut to a length of about three-eighths inch and 70% of prebeaten, fibrillated paper-making cellulose fibers derived from sulfite pulp, is mixed with the dispersion until the fibers are uniformly distributed therein. Sufficient concentrated sulfuric acid is added, with mixing, to give a stock which, after dilution to 0.020.002% fiber concentration, has a pH of 6-7. The stock is sheeted on a paper machine. The sheet is passed over the drying cans at 145 C., and then to the calender rolls between which it is pressed at 1000 lbs/sq. in. In the final product, the regenerated cellulose fibers are bonded at their points of intersection by the discontinuous cellulose ether. The paper sheet is relatively porous and exhibits good resistance to tearing or bursting in the wet state. The paper is more readily wettable than a similar paper formed entirely of fibrillated cellulose fibers.

The paper products of the invention are clear and substantially colorless. Those paper products comprising regenerated cellulose fibers as the non-fibrillating synthetic or artificial fibers do not exhibit the yellowish cast which is usually encountered when regenerated cellulose is treated with alkaline media.

Although the invention has been illustrated in the specific examples in connection with the production of paper sheets, it is obvious that paper articles having a three-dimensional shape can be obtained. For example, the charge comprising the dispersion of the fibers carrying the precipitated cellulose ether can be fed into a vat from which the fibers are sucked against a perforated mold of any shape by use of vacuum, to form a layer of desired thickness, the layer being subsequently dried and pressed to form a product having the contours of the mold.

Paper produced in accordance with this invention can be reclaimed without difficulty, since the water-insoluble, alkali-soluble ether is dissolved in the digester.

Since certain changes in carrying out the above process, and certain modifications in the paper articles which embody the invention may be made without departing from its scope, it is intended that all matter contained in the above description shall be interpreted as illustrative and not in a limiting sense.

I claim:

1. As a new article of manufacture, a porous paper product comprising a multiplicity of randomly intermingled discontinuous fibers all of which are organic, non-fibrillating, extruded synthetic, artificially formed fibers having a length of from one-thirty second to thirteen-thirty seconds of an inch and bonded at their points of intersection in the paper by a water-insoluble, alkalisoluble hydroxyethyl cellulose which occurs in the paper in the form of discrete particles of larger than colloidal size.

2. A porous paper product as in claim 1, in which all of the fibers are regenerated cellulose fibers having a length of from one-thirty second to thirteen-thirty seconds of an inch and the water-insoluble alkali-soluble hydroxyethyl cellulose is soluble in aqueous alkali solutions of from 1% to 8% concentration.

3. As a new article of manufacture, a porous paper sheet comprising a multiplicity of discontinuous fibers randomly intermingled in the sheet, all of the fibers being organic, non-fibrillating, extruded, synthetic, artificially formed fibers having a length of from one-thirty second to thirteen-thirty seconds of an inch and bonded at their points of intersection in the sheet by a water-insoluble, alkali-soluble, hydroxyethyl cellulose which occurs in the sheet in the form of discrete particles of larger than colloidal size, the sheet being free of fibrillae.

4. A method of making porous fibrous products com-,7

prising a multiplicity of randomly intermingled discontinuous fibers which comprises distributing fibers all of which are organic, non-fibrillating, extruded, synthetic, artificially formed fibers having alength of from onethirty second to thirteen-thirty seconds of an inch through an aqueous alkaline medium of less than 1% alkali concentration and containing, on the weight of the water of said medium, between 0.02% and 0.5% of a waterinsoluble hydroxyethyl cellulose which is soluble in aqueous alkaline solutions of between 1% and 8% alkali concentration, said hydroxyethyl cellulose being dispersed in the aqueous alkaline medium of less than 1% alkali concent ation in the form of colloidal gel particles, acidifying the dispersion to precipitate the hydroxyethyl cellulose on the surfaces of the fibers in the form of discrete particles of larger than colloidal size, diluting the acidified dispersion with water, separating the fibers carrying the precipitated hydroxyethyl cellulose particles from the aqueous dispersion to obtain a paper product of predet'ermined shape and thickness, and drying and pressing the product.

5. A method of making a porous, readily wettable paper product comprising a multiplicity of randomly intermingled discontinuous fibers and free from fibrillae which comprises distributing discontinuous fibers all of which are non-fibrillating regenerated cellulose fibers having a length of from one-thirty second to thirteenthirty seconds of an inch through an aqueous alkaline medium of less than 1% alkali concentration and containing, on the weight of the water of the medium, between 0.02% and 0.5% of a water-insoluble hydroxyethyl cellulose which is soluble in aqueous alkaline solutions of between 1% and 8% alkali concentration, the hydroxyethyl cellulose being dispersed in the aqueous alkaline medium of less than 1% alkali concentration in the form of colloidal gel particles, adding sulfuric acid to the dispersion to precipitate the dispersed hydroxyethyl cellulose on the surfaces of the fibers in the form of particles of larger than colloidal size, diluting the dispersion to a pH of about 6 to 7, continuously feeding the dispersion to the screen of a paper-making machine to form a sheet, and drying and pressing the sheet to obtain an all regenerated cellulose fiber paper sheet in which the fibers are bonded at their points of intersection by the hydroxyethyl cellulose essentially all of which occurs in the sheet in the form of the particles of larger than colloidal size.

References Cited in the file of this patent UNITED STATES PATENTS 1,829,585 Dreyfus et al. Oct. 27, 1931 1,898,601 Shoemaker Feb. 21, 1933 2,033,481 Richter Mar. 10, 1936 2,069,763 Jones Feb. 9, 1937 2,087,237 Bolton July 20, 1937 2,236,545 Maxwell et al. Apr. 1, 1941 2,297,248 Rudolph Sept. 29, 1942 2,370,517 Bass et al. Feb. 27, 1945 2,414,833 Osborne Ian. 28, 1947 2,477,000 Osborne July 26, 194 2,533,145 Schorger Dec. 5, 1950 2,565,941 Barnard Aug. 28, 1951 2,572,932 Horsey et al. Oct. 30, 1951 2,626,214 Osborne June 20, 19 3 FOREIGN PATENTS 462,254 Great Britain Mar. 4, 1937 505,200 Great Britain May 8, 1939 530,293 Great Britain Dec. 9, 1940 

1. AS A NEW ARTICLE OF MANUFACTURE, A POROUS PAPER PRODUCT COMPRISING A MULITPLICITY OF RANDOMLY INTERMINGLED DISCONTINUOUS FIBERS ALL OF WHICH ARE ORGANIC NON-FIBRILLATING, EXTRUDED SYNTHETIC, ARTIFICIALLY FORMED FIBERS HAVING A LENGTH OF FROM ONE-THIRTY SECOND TO THIRTEEN-THIRTY SECONDS OF AN INCH AND BONDED AT THEIR POINTS OF INTERSECTION IN THE PAPER BY A WATER-INSOLUBLE, ALKALISOLUBLE HYDROXYETHYL CELLULOSE WHICH OCCURS IN THE PAPER IN THE FORM OF DISCRETE PARTICLES OF LARGER THAN COLLOIDAL SIZE. 